Make calling term_nopaste() a cross-platform feature.

It was one of those things that went in ages ago on Windows and never
got replicated in the Unix front end. And it needn't be: ldisc.c is a
perfect place to put it, since it knows which of the data it's sending
is based on a keystroke and which is automatically generated, and it
also has access to the terminal context. So now a keypress can
interrupt a runaway paste on all platforms.

[originally from svn r10025]